Nick Clegg cannot have enjoyed his first day back in parliament yesterday, bashed about from all sides over his bill to redraw parliamentary boundaries and hold a referendum on the alternative vote. Jack Straw, by contrast, had a lot of fun indeed on what may be his final significant Commons frontbench appearance. He mocked the Liberal Democrats for supporting a voting system which not long ago they derided as a miserable compromise. He accused the coalition of planning to fix elections to its own advantage. He led his party in opposition to Mr Clegg's bill and found many Tory MPs were prepared to lend him their support. He will have been pleased with his day's work. But he was on the wrong side.

Mr Clegg spoke for progress; Mr Straw for reaction. Labour has got its teeth into the government, but at some cost to the cause of reform. Earlier this year Labour wanted a vote on the alternative vote, for good reasons. Now it is against a bill that will allow one. Its next leader is likely to find himself in the position of trying to campaign for a yes vote in a referendum that he opposed in the Commons. Voters will surely be sceptical and the referendum less likely to be won as a result.

The Labour answer is to point to the bill, which does much more than allow the alternative vote. The party has set its heart against a piece of legislation that it sees as Tory gerrymandering, cutting the number of MPs by 50 and compelling the Boundary Commissions to redesign every seat so that they contain roughly equal numbers of voters. Labour is also opposed to the date of the proposed referendum, on the spuriously disruptive grounds that it is a bad idea to have more than one vote on the same day. This latter objection is nonsense; the former has some merit, but not enough to justify trying to block the whole bill, rather than just voting against specific clauses. There is nothing inherently unfair about giving votes equal weight in all parts of the country, even if there are arguments about how it is to be done. Indeed, it could be said that the current situation involves an element of formal rigging, intentionally overstating the representation of Wales and Northern Ireland. There are decent reasons for this, but it is explicitly unfair all the same. Labour, which at the moment gains most from the existence of smaller than average constituencies, ought to pause before charging its opponents with rigging elections.

The more pressing issue, however, is the alternative vote. Barring an unlikely parliamentary catastrophe this week, there will be a referendum next year. It would be best if it were won. Last year, after the parliamentary expenses debacle, Labour came out in support of AV. "Whether by doing nothing, or by design, we retreat into a discredited old politics, leaving power concentrated in the hands of the old elites," argued Gordon Brown, before attempting to introduce a parliamentary referendum of his own in a much wider bill (a tactic Mr Straw condemned yesterday). The party was right to back AV then, and it should back it still. Nothing, in principle, has changed – only parliamentary arithmetic and Labour's desire to punish the Lib Dems for joining the Conservatives in government.

The alternative vote is an imperfect compromise, but it is better than first past the post. Evidence from the recent Australian election – held under AV – can be twisted to support almost any argument: but Labor hung on to power, in part, thanks to second-preference votes from the Greens, as it would not have done under first past the post. The same thing could help the Labour in Britain, but only if the party throws its full weight behind the campaign for electoral change. The alternative is to cower, and denounce the referendum as a Tory trick, professing still to want AV, while knowing it will never come. A rare and great opportunity for progress is being missed.